Table 4 Materials, application and clearance of the cleaning options tested on mock-ups

Soft particle blasting

 Wheat starch (20 µm) 500 g/L. pH 4.0–7.5

Soft particles used as dispatched. Chambers filled 2/3 full to ensure that the airflow can swirl the particles inside the chambers

Based on initial cleaning trials, a 1.2 size nozzle was directed at a 40–45° angle to the surface with the working pressure set to minimum (c. 1.8 bar/26.1 psi). Application: intermittent blasts by switching the hand switch on and off every two to three seconds to achieve an even particle stream. Stage two with Arbocel® A, nozzle distance to surface: G mock-up: 6–7 cm, Pu mock-ups: 3–4 cm, Pd mock-ups 7 cm

Cleaning particles and displaced soiling removed with an air gun followed by brushing with soft brushes

 Arbocel® A (40 µm) 190–250 g/L. pH 5.0–7.0

 Arbocel® B (120 µm) 150–182 g/L. pH 5.0–7.0

CO₂-snow

 Liquid and gas CO₂ source

Cover gas heated for at least 20 min prior to cleaning

Compound Venturi and surrounding cover gas nozzle was introduced at 90° angle to the plane of the sample and c. 30 cm distance to gradually reduce the sample surface temperature and avoid condensation [14]. The nozzle was brought closer to the surface at a 40–45° angle to c. 3–5 cm from the sample surface. Continuous CO₂-snow flow for c. 5 s with side-to-side sweeping action

Not required because CO₂-snow particles sublimate. Progressing with cleaning in the direction of the CO₂ stream avoids ejecting soiling onto already-cleaned areas

 The cover gas was nitrogen

Nanorestore Gels®

 MWR, HWR, Peggy 5, Peggy 6

The nano-gels come pre-made in a plastic bag with a single sheet of c. 10 × 15 cm² immersed in demineralised water. Nano-gels are immersed for 24 h in DI water prior to use or before loading with cleaning solutions. Nano-gels are immersed in selected cleaning solutions for at least 12 h prior to use

Nano-gels were blotted using Evolon® tissues to remove excess water. Light finger pressure was applied to optimise adhesion when the gel was applied to the surface. The gels were gently peeled off the surface. Stage one: free liquid trials with manufactured swabs on M1. Cleaning trials on M1 with 3.5 × 2 cm² pieces of nano-gels loaded with DI, adjusted waters, chelating agents, and surfactants and applied to each sample type for one minute. Stage two: Pieces of 5.5 × 5 cm² nano-gel applied to M2, loaded with optimal system based on observations from Stage one and applied for one minute. G mock-ups: Nanorestore® MWR gel, 0.5% w/v citric acid/NaOH pH 6.5, 4.5 mS/cm. Clearance: water adjusted to pH 6.5. Pu mock-ups: Peggy 6, 1% w/v citric acid/NaOH pH 6.5, 9.2 mS/cm. Clearance: Peggy 6 with water adjusted to pH 6.0 Pd mock-ups: MWR, water adjusted to pH 6.0

Not required for pure hydrogels. Nano-gels loaded with chelating agents or surfactants require a second clearance step using the same gel type loaded in water

Aqueous cleaning solutions:

DI water: c. pH 6.9, 2 µm/cm

Adjusted waters (acetic acid and dilute ammonium hydroxide): pH 5.0–7.5, 1000 µm/cm. Buffered waters: MES pH 5.5, MES pH 6.0, Bis–tris pH 6.5, TRIS pH 7.5 (only in free liquid trials). Chelating agents: TAC: 0.5%–1% w/v, pH 7.1, 4.2–8.9 mS/cm, 0.5%–1% w/v Citric acid/NaOH: pH 5.5–6.5, 4.4–9.4 mS/cm. Nonionic surfactants: ECOSURF™ EH 6 (HLB 10.8) and ECOSURF™ EH9 (HLB 12.5)

 Dry, white, latex-free polyurethane (PU) sponge

Used as received, without modification

Parallel strokes applied with gentle finger pressure in vertical, horizontal, and diagonal directions. G mock-ups: 12 strokes, Pu mock-ups: 18 strokes, and Pd mock-ups: 9 strokes

Not required for PU dry sponges